I sealed my magnesium wheels with I believe a
zinc phosphate paint from a marine store. Comes in either the puke green or yellow color. Moeller is the company. Keeps the air from leaking out. Honestly I didn't know aluminum had that issue.
As far the price on these wheels. If I had an original Cobra and needed a set I would be very happy to part with $8000.00 to complete my car if this is what they needed. These wheels are only getting harder to find and they only come around once in a while. The price is relative to the car they are going on. Yes about five years ago similar wheels would be seen for $4000 or $5000. If you need a set and want to wait to see if you can get a better deal down the road by all means wait. Probably the guy with 500k car is not going to wait....